The present invention relates to a semiconductor device and, more particularly, to a high-power monolithic microwave integrated circuit device using a compound semiconductor and to a monolithic microwave/millimeter-wave array antenna module.
In recent years, a high-power compound semiconductor device has been rapidly developed. A high-power GaAs FET of several tens W in a C band and a high-power HBT (heterojunction bipolar transistor) of several W in an X band have been developed. In these high-power devices, in order to keep reliability, the temperature of an operating layer must be suppressed to be a predetermined value or less. For this reason, heat resistance must be decreased. More specifically, in an HBT having a large amount of current per unit area (the amount is about three times that of a GaAs FET), a power density is increased to almost three times that of the GaAs FET. Therefore, the heat resistance must be 1/3 or less that of the GaAs FET.
Methods of largely decreasing heat resistance are roughly classified into two types of methods. The first method is a method of thinning a semiconductor substrate. FIG. 7A is a sectional view showing the first conventional method. In FIG. 7A, an HBT comprising an n-type GaAs collector layer 30, a p.sup.+ -type GaAs base layer 31, and an n-type AlGaAs emitter layer 32 is formed on a semi-insulating GaAs substrate 21. Reference numeral 25 denotes an emitter electrode of the HBT. A collector electrode 27 of the HBT is connected to an output microstrip conductor 23, and a base electrode 26 of the HBT is connected to an input microstrip conductor 24. A ground metal layer 22 is formed on the lower surface of the GaAs substrate 21. In FIG. 7A, in order to decrease the heat resistance of the substrate to 1/3, a thickness t of the substrate must be decreased to 1/3. At this time, since an electrostatic capacitance between the microstrip conductors 23 and 24 and the ground metal layer 22 is increased to be about three times, the characteristic impedance is decreased to about 1/.sqroot.3. Therefore, in order to keep the characteristic impedance, the widths of the microstrip conductors 23 and 24 must be decreased to 1/3. In this case, since the series resistance of this line is increased, the transmission loss of the microstrip line is disadvantageously increased. In addition, a thinned substrate is easy to break and therefore difficult to handle.
As the second conventional method of decreasing heat resistance, a flip-chip structure shown in FIG. 7B is used. In FIG. 7B, an HBT comprising an n-type GaAs collector layer 30, a p.sup.+ -type GaAs base layer 31, and an n-type AlGaAs emitter layer 32 is formed upside down. The emitter electrode of the HBT is directly connected to a ground metal block 41 by a thermocompression bonding method. The ground metal block 41 radiates heat. A base electrode 36 is connected to an input microstrip conductor 39 arranged on an alumina substrate 40 formed on a ground metal block 41, and a collector electrode 37 is connected to an output microstrip conductor 38 arranged on the alumina substrate 40 formed on the ground metal block 41. When the conventional flip-chip structure is used, although the heat resistance can be extremely decreased, the input and output microstrip circuits cannot be formed to be monolithic.
That is, in the conventional method in FIG. 7A, although a monolithic HBT including circuits can be obtained, transmission loss is disadvantageously increased in order to decrease heat resistance. In the conventional method in FIG. 7B, although heat resistance can be largely decreased, a monolithic IC cannot be obtained.
In addition, when not only a transmitting/receiving module such as the microwave/millimeter-wave radar device but an antenna element are monolithically integrated, about several hundreds antenna elements each having a transmitting/receiving module can be arranged on a large-diameter GaAs wafer in an array form, and a monolithic microwave array antenna module for a phased array radar can be obtained. The above prior art is summarized in the Microwave Journal, July, 1986, p. 119.
FIG. 8 is a sectional view showing a conventional monolithic microwave array antenna module. In FIG. 8, an active element circuit 162 including a low-noise amplifier, a mixer, a switch, a power amplifier, a phase shifter, and an A/D converter, and a microstrip dipole antenna 122 are formed on the surface of a semi-insulating GaAs substrate 101 having a ground metal layer 102 on its lower surface.
In addition to the problem of heat resistance described above with reference to FIGS. 7A and 7B, a problem to be solved in the conventional method in FIG. 8 will be described below. The relative dielectric constant .epsilon.r of GaAs is 12.7, and the wavelength shortening ratio of GaAs is about 0.33. Thus, the width of a half-wave microstrip dipole antenna 22 must be set to be 1.25 mm at a frequency of 40 GHz. However, a wavelength .lambda.g/2 of a half-wave of a 40-GHz electromagnetic wave is 3.75 mm in a free space, and this value is largely different from the above value of 1.25 mm. It causes high current density. Therefore, energy input to the microstrip dipole antenna is not effectively radiated to the space, because of antenna conductor loss.